1. Field of the Invention
This invention relates to safety systems for controlling subterranean wells, more particularly to safety valve assemblies for controlling the flow in a subterranean well comprising concentric inner and outer conduits with safety valves being operable to prevent flow through the inner tubing and through the annulus between inner and outer concentric tubing members.
2. Description of the Prior Art
In subterranean oil and gas wells, the reservoir pressure is often sufficient to cause produced petroleum fluids to flow naturally to the surface of the well. In other wells, the pressure is initially insufficient to permit production of fluids in this manner or the pressure becomes insufficient after some period of continued operation. It then becomes necessary to provide some means to artificially stimulate production of the wells. One means of furnishing such artificial stimulation is through artificial gas lift techniques. Gas may be continuously injected from the surface of the well to the formation or it may be intermittently injected to lift produced fluids to the surface of the well. When artificial gas lift is necessary, it is standard practice to install conventional gas lift control mandrels, injection mandrels and conventional gas lift valves in the oil well. Where production from the well has previously relied upon the natural reservoir pressure, it has heretofore been standard practice to pull the existing production tubing string and install conventional gas lift control mandrels, injection mandrels, and gas lift valves in order to produce by artificial stimulation means. This practice necessitates the complete removal of the existing production string, however. In addition to the complete reworking of the existing production string, there are other methods of artificially stimulating a well to provide artificial gas lift. One example would be to use a macaroni string tubing inserted into the existing tubing. Gas inserted through this macaroni tubing would be conveyed to the formation and would provide a means for producing fluid through the annulus between the inner tubing and the outer production tubing.
Through tubing injection with annulus production, even with the addition of an inner tubing member, still requires the use of appropriate safety valves to prevent unrestricted flow through the tubing in the annulus in case of some catastrophe. The present invention contemplates the use of a conventional safety valve to restrict flow through the inner conduit. However, in addition to preventing flow through the inner conduit, it is also necessary to prevent flow along the normal production path in the annulus between the inner and outer conduits. An annulus safety valve must be employed to prevent flow in this manner. Conventional wireline retrievable annular safety valves for providing fail-safe closure for wells in which annulus flow is required are shown on pages 778-781 of the 1980-1981 edition of the Composite Catalog of Oil Field Equipment & Services published by World Oil. These conventional annulus safety valves are generally used to provide a fail-safe seal in the annulus between the production tubing and the outer casing in a conventional well. These annulus safety valves generally employ an axially movable sleeve member which moves to open a port extending radially through the sides of the nipples when subjected to control line pressure. Other annulus safety valves are shown on pages 5383-5387 of the 1978-1979 Composite Catalog of Oil Field Equipment & Services. An axially movable piston member which moves when subjected to hydraulic control line pressure has also been employed to open and close radially extending ports in at least one of the annulus type safety valves depicted therein.